All stars are formed from nebulae (the plural of nebula). Nebula is a term for a cloud of gas, and stars form from gas. Stars more massive than ~ 6 solar masses are expected to supernova, stars less massive than this (like our Sun, of course) become white dwarfs. After a supernova, there may be nothing left, or there could be a remnant: either a neutron star or a
black hole. If the remnant is more massive than around 3 solar masses it will probably end up as a
black hole. Stars are smallest when they are burning hydrogen into helium, which is what stars do during most of their lifetimes. Stars in this stage are sometimes called dwarfs. There are also two other kinds of "dwarfs": white dwarfs are burned-out stars mentioned above (the Learning
Centre has more info on these), and brown dwarfs are stars which never accumulated enough mass to start burning hydrogen.

Stars produce energy primarily by nuclear reactions in their deep interiors. The nuclear reactions produce very high-energy particles and gamma-rays, but these can't escape easily through the outer layers of the star; they must scatter many times on their way out. This scattering process, plus the total amount of energy produced by the nuclear reactions, are what determines the spectrum of radiation escaping the star. This spectrum is pretty well described by a shape called a 'black body', which also applies to many terrestrial situations, such as an incandescent light bulb. For a black body the temperature and
colours are related in a simple way.

The locations of most (about 90%) of the
stars on this diagram are clustered in a relatively thin curved band that
stretches from the upper left to the lower right. This band is called the
main sequence. Main sequence stars have spectral types ending with the
label V, indicating relatively low luminosity. The stars at the upper left
of the main sequence are hot (35,000 K) type O5 hot blue giant stars with
luminosities more than 105 times that of the Sun, while those
at the lower right are cool (3000 K) type M5 red dwarf stars with
luminosities less than 10-4 times that of the Sun. The range in
photospheric temperature is slightly more than a factor of 10, but the net
range in luminosity is enormous more than 109 (1 billion). In
the upper right of the diagram are the red giant stars; with radii ranging
up to more than 100 times the Sun's radius; they are the biggest stars on
the diagram. On the lower left are the white dwarf stars; with radii less
than 0.1 times the Sun's radius, they are the smallest stars on the
diagram.

Cluster Type

Number of Stars

Interstellar gas nearby?

Brightest Stars

White Dwarfs?

Open

102 - 103

Yes

Blue giants

Few

Globular

105 - 106

No

Red giants

Many

Supernovas

After a Supernova........and before.

Supernovae are divided into two basic physical types:
Ia. These result in some binary star
systems between a red giant and a white dwarf. In such a system, mass
flows from the red giant to the white dwarf. Eventually, so much mass
piles up on the white dwarf that it can no longer support itself and it
collapses.
II. These supernovae occur at the end of a massive star's lifetime, when its nuclear fuel is exhausted and it
is no longer supported by the release of nuclear energy. If the star's
iron core is massive enough then it will collapse and become a supernova.

However, these types of supernovae
were originally classified based on the existence of hydrogen spectral
lines: Type Ia do not show hydrogen lines, while Type II do. In general
this observational classification agrees with the physical
classification outlined above, because massive stars have atmospheres
(made of mostly hydrogen) while white dwarf stars are bare. However, if
the original star was so massive that its strong stellar wind had
already blown off the hydrogen from its atmosphere by the time of the
explosion, then it too will not show hydrogen spectral lines. These
supernovae are often called Type Ib supernovae, despite really being
part of the Type II class of supernovae.

What Causes a Star to Blow Up? Gravity
gives the supernova its energy. For either type of supernova, mass flows
into the core, from a companion star (I) or by the continued making of
iron from nuclear fusion (II). Once the core has gained so much mass
that it cannot withstand its own weight, the core implodes. This
implosion can usually be brought to a halt by neutrons, the only things
in nature that can stop such a gravitational collapse. Even neutrons
sometimes fail depending on the mass of the star's core. When the
collapse is abruptly stopped by the neutrons, a bounce occurs, thus
turning the implosion into an explosion. ka-BOOM!!!

When the core is lighter than about 5
solar masses, it is believed that the neutrons are successful in halting
the collapse of the star creating a neutron star. Neutron stars can
sometimes be observed as pulsars or X-ray Binaries. When the core is
heaver (Mcore > ~ 5 solar masses), nothing in the known universe is
able to stop the core collapse, so the core completely falls into
itself, creating a black hole, an object so dense that even light cannot
escape its gravitational grasp. The core is only the very small centre of an extremely large star that for many millions of years had been
making many (but not all) of the elements that we find here on Earth.
When a star's core collapses, an enormous blast wave is created with the
energy of about 1028 mega-tons. This blast wave ploughs the star's
atmosphere into interstellar space, propelling the elements outward as
the star becomes a supernova remnant. In addition to making elements,
supernovae scatter the elements (made by both the star and supernova)
out in to the interstellar medium. These are the elements that make up
stars, planets and everything on Earth including ourselves.

Although many supernovae have been
seen in nearby galaxies, supernova explosions are relatively rare events
in our own Galaxy, happening once a century or so on average. The last
nearby supernova explosion occurred in 1680, It was thought to be just a
normal star at the time, but it caused a discrepancy in the observer's
star catalogue which historians finally resolved 300 years later, after
the supernova remnant (Cassiopeia A) was discovered and its age
estimated. In 1987 there was a supernova explosion in a companion galaxy
to the Milky Way. Supernova 1987A, which is shown at the top of the
page, is close enough to continuously observe as it changes over time
thus greatly expanding astronomers' understanding of this fascinating
phenomenon. A supernova remnant (SNR) is the remains of a supernova
explosion. SNRs are extremely important for understanding our Galaxy.
They heat up the interstellar medium, distribute heavy elements
throughout the Galaxy, and accelerate cosmic rays.

Supernova can affect us in some
important ways. First and foremost, we and much of the Earth are made of
the material supernovae created. According to current theories about the
formation of the Universe, all of the original material in the Universe
was hydrogen and helium, with very slight traces of some other
materials. All the stuff we, and the Earth around us, are made of, like
iron and oxygen and carbon, has come from that initial material being
fused to form heavier elements in the cores of stars. But the heaviest
elements, like iron, are only formed in the massive stars which end
their lives in supernovae. Our blood has iron in the haemoglobin which is
vital to our ability to breath. So without supernovae, most forms of
life on Earth, including us, would not be possible. And much of the
material the Earth is made of would not exist.

Supernovae also create shock waves
through the interstellar medium (the stuff between stars), compressing
material there. Astronomers believe that these shock waves are vital to
the process of star formation, causing large clouds of gas to collapse
and form new stars. No supernovae, no new stars. Supernovae throw much
of the material from their parent star back out into the interstellar
medium, changing its chemical composition. This adds many elements to
the interstellar medium which were not present before, or were only
present in trace amounts. Other less massive stars also enrich the
interstellar medium, but lack many of the heavier elements. The gradual
enrichment of the interstellar medium with heavier elements has made
subtle changes to how stars burn: the fusion process in our own Sun is
moderated by the presence of carbon. The first stars in the Universe had
much less carbon and their lives were somewhat different from modern
stars. Stars which will be formed in the future will have even more of
these heavier elements and will have somewhat different life cycles. So
supernovae play a very important part in this chemical evolution of the
Universe.

A supernova has to be within 10
parsecs (30 light years) or so to be dangerous to life on Earth. This is
because the atmosphere shields us from most dangerous radiations.
Astronauts in orbit may be in danger if a supernova is within 1000
parsecs or so. No stars currently within 20 parsecs will go supernova
within the next few million years. There are some indirect effects,
though, which are harder to evaluate: the possible effects on the Earth
ozone layer is listed in the article above. Additionally, according to
one calculation, the neutrino flux from a nearby supernova might heat up
the Sun.

The minimum temperatures for nuclear reactions
to occur are listed in the table below:

Reaction

Temperature (K)

Hydrogen burning: 4 H ®
He

(1 - 5) x 107

Helium burning: 3 He ®
C; He + C ® O

2 x 108

Carbon/Oxygen burning: C + O ®
all other elements

7 x 108 - 2 x 109

Supernova
Remnants

Shell-type remnants:
The Cygnus Loop (above left) is an
example of a shell-type remnant. As the shock wave from the supernova
explosion ploughs through space, it heats and stirs up any interstellar
material it encounters, thus producing a big shell of hot material in
space. We see a ring-like structure in this type of SNR because when we
look at the edge of the shell, there is more hot gas in our line of sight
than when we look through the middle. Astronomers call this phenomenon
limb brightening.

Crab-like remnants:
These remnants (also called plerions)
resemble the Crab Nebula (above right). These SNRs are similar to
shell-type remnants, except that they contain a pulsar in the middle that
blows out jets of very fast-moving material. These remnants look more like
a "blob" than a "ring."

Composite Remnants:
These remnants are a cross between the
shell-type remnants and crab-like remnants. They appear shell-like,
crab-like or both depending on what part of the electromagnetic spectrum
one is observing them in. There are two kinds of composite remnants --
thermal and plerionic.

Thermal composites:

These SNRs appear shell-type in the
radio waveband (synchrotron radiation). In X-rays, however, they appear
crab-like, but unlike the true crab-like remnants their X-ray spectra have
spectral lines, indicative of hot gas.

Plerionic composites:

These SNRs appear crab-like in both
radio and X-ray wavebands; however, they also have shells. Their X-ray
spectra in the centre do not show spectral lines, but the X-ray spectra
near the shell do have spectral lines.

Naturally, if the supernova explosion was
recorded in history, as is the case of many SNRs less than a few thousand
years old, we know the age of the corresponding SNR. However, sometimes
historians are not certain if a recorded guest star was a
supernova or was the same supernova as a corresponding remnant. It is
therefore important to be able to estimate the age of SNRs. An easy way to guess the age of a SNR is to measure the temperature of
the hot gas using X-ray spectroscopy. From this observation we can
estimate the velocity of the shock wave, and then infer the age from the
shock velocity. This is easy to do, but not very accurate, because there
are a number of complicated processes that can heat up or cool down the
gas which are independent of shock velocity.

Supernova remnants greatly impact the ecology of the Milky Way. If it
were not for SNRs there world be no Sun or Earth. Every element found in nature, except for hydrogen and helium, was made
in either a star or a supernova explosion. So, how did these elements come
to be on Earth? Through the action of supernova remnants. The gas that fills the disk of the Milky Way is called the interstellar
medium ( ISM). In the parts of the galaxy where the ISM is most dense (for
example in the Galaxy's spiral arms), the ISM gas can collapse into
clumps. Clumps that are above a critical mass (somewhere between the mass
of Jupiter and the Sun) will ignite nuclear fusion when the clumps
gravitationally collapse, thus forming stars. Therefore, the chemical
composition of the ISM becomes the chemical composition of the next
generation of stars. Because, supernova remnants mix supernova ejecta (including the newly
formed elements) into the ISM, if it were not for supernova remnants our
solar system, with its rocky planets, could never have formed.

In addition to enriching our Galaxy with heavy elements, supernova
remnants add a great deal of energy to the ISM (1028 mega-tons per
supernova). As the shock wave moves outward it sweeps across a large
volume of the ISM, impacting the ISM in two primary ways: The shock wave heats the gas it encounters, not only raising the
overall temperature of the ISM, but making some parts of the Galaxy hotter
than others. These temperature differences helps to keep the Milky Way a
dynamic and interesting place. The shock wave accelerates electrons, protons, and ions to velocities very close to the speed of light. This
phenomenon is very important, because the origin of the cosmic rays is one
of great outstanding problems in astrophysics. Most astronomers believe
that most cosmic rays in our Galaxy used to be part of the gas in the ISM,
until they got caught in a supernova shock wave. By rattling back and
forth across the shock wave, these particles gain energy and become cosmic
rays.

Dwarfs

A black dwarf is a white dwarf that has cooled down enough that it no longer emits light. See the Imagine the Universe science pages for the differences between white dwarfs, neutron stars, and black holes.
A white dwarf is formed when a star has burned all of its original hydrogen and helium fuel to elements such as carbon, nitrogen and oxygen. If the star doesn't have enough mass, the pressure at its
centre is too low to burn these elements further, and so it no longer produces heat. It is, however, still hot from the earlier burning stages, so it still glows for a while until it cools down. It takes tens to hundreds of billions of years for it to cool down entirely, and the Universe hasn't been around that long--the oldest stars are between 10 and 20 billion years old. Therefore there are no black dwarfs yet, but there will be in the future.

Brown dwarfs are small size objects, believed to result from condensations of fragments of molecular clouds. A brown dwarf has a small mass, too low to ignite nuclear fusion. Such a star, would be small, not much larger than Jupiter, and warm from its contraction. It would emit copious infrared radiation, thus the name
brown dwarf.

Accretion disks arise when material (usually gas) is being transferred from one celestial object to another. "accretion" means collecting of additional material. Two major places where astronomers see accretion disks are in binary star systems (two stars orbiting each other) and active galactic nuclei.
I will discuss an accretion disk in a binary star system, but the basic ideas are the same for all cases. If one star in a binary system is a compact object such as a very dense white dwarf star and the other star is a normal star like the sun, the white dwarf can pull gas off the normal star and accrete it onto itself. Since the stars are revolving around each other and since angular momentum must be conserved, this gas cannot fall directly onto the white dwarf, but instead spirals in to the white dwarf much like water spirals down a bathtub drain. Thus material flowing from the normal star to the white dwarf piles up in a dense spinning accretion disk orbiting the white dwarf. The gas in the disk becomes very hot due to friction and being tugged on by the white dwarf and eventually loses angular momentum and falls onto the white dwarf. Since this hot gas is being accelerated it radiates energy, usually in x rays which astronomers detect and use to identify and study accretion disks.

Giants

Towards the end of a star's life, the temperature near the core rises and this causes the size of the star to expand. This is the fate of the sun in about 5 billion years.
Stars convert hydrogen to helium to produce light (and other radiation). As time progresses, the heavier helium sinks to the
centre of the star, with a shell of hydrogen around this helium centre core. The hydrogen is depleted so it no longer generates enough energy and pressure to support the outer layers of the star. As the star collapses, the pressure and temperature rise until it is high enough for helium to fuse into carbon, i.e. helium burning begins. To radiate the energy produced by the helium burning, the star expands into a Red Giant.

Binary Stars

In essence, a binary-star system emerges out of a cloud of gaseous material collapsing and forming more than a single star at the same time in a small proximity. This type of a collapsing event does not necessarily form only two stars -- it can form more than two, but it all depends on their unique environment in which stars form.

Also it is most unlikely for a single star to capture another star in a typical stellar space. When two stars encounter, they tend to swing by each other and almost never captures one to another by their own gravitational field. We are not going to explain why it is so, but you will need more than two stars (in fact, many stars) to do just that. Some cases of binary-capture may have been seen in a place like globular clusters where a million of stars are found in a very tiny volume of space.

There are stable orbits for planets in binary
star systems. There are various stability criteria which say when an orbit
is stable. One such criteria (and I don't know the actual numbers) says
that if all orbits are circular and the stars are the same size, then the
planet must orbit one of them at less than /some fraction/ of the
inter-star distance, or must orbit both combined at more than /whatever/
times the inter-star distance. Figure-eight orbits are unstable, and can
eject the planet from the system.

If you have two Sun-like stars at the centre of
the system, a planet would be the same temperature as Earth if it were at
sqrt(2) = 1.4 astronomical units away, rather than Earth's 1 AU. This
distance is closer than Mars's orbit (1.6 AU). Most stars are dimmer than
our Sun, so the orbit could be even smaller

The solar-like stars 16 Cygni B and 55 Cancri A have been found to have Jupiter-size extrasolar planets orbiting them. So we do have indirect proof, through Doppler spectroscopy methods (Marcy & Butler, SFSU, Lick Observatory), that planets indeed form in binary systems.
The formation mechanisms for forming stars and planets are very different. Planets require accretion to form, specifically accretion in a protoplanetary disk around a young star. Stars can form from the collapse of a molecular cloud core on their own, however planets can only form in the disk around a star. (Pulsar planets are likely formed "posthumously" around pulsars, and are a different beast all together). The main problem with forming planets in multiple star systems is dynamic ejection... stars can simply toss planetesimals out of the system all together (or even accrete them). An example of this is the Kirkwood gaps in the asteroid belt where Jupiter doesn't allow asteroids to exist in certain orbits, and conversely it "shepherds" asteroids in to certain other orbits. A companion star would have a similar effect, except there would be a lot less "shepherding" orbits. The vast majority of binary stars have eccentric orbits. It is difficult for bodies to exist in a system with two very massive bodies in an eccentric orbit. They can only exist very close to each star, or very far from both stars.

An excellent example is the nearby solar-like stars Alpha Centauri A and B. They orbit each other at an average distance of 23 AU, however the eccentricities of each orbit bring them to as close to 11 AU and as far as 35 AU. Numerical simulations by Paul Weigert at University of Toronto have shown that each star has a "safe zone" about 3 AU in radius in which planets could safely survive for billions of years. Objects placed further out from each star than about 3 AU are dynamically ejected in a matter of millions of years or less. Alpha Cen A is about 1.5 times as luminous as our Sun, and Alpha Cen B is about .45 times as luminous as our Sun, and if you do the simple physics, one can see that a "habitable zone" exists around BOTH stars within the 3 AU dynamic "safe zone." Indeed, it could be possible that BOTH Alpha Cen A and B have planets conducive to life. Theoretical models age them anywhere from 3-8 Gyr... plenty of time for life to develop if the planets have the right conditions...

Cataclysmic
Variables

Cataclysmic variables (CVs)
are binary star systems which have a white dwarf and a normal star
companion. They are typically small, the entire binary system usually has
the size of the Earth-Moon system with an orbital period in the range 1-10
hrs. The white dwarf is often referred to as the primary star, and the
normal star as the companion or the secondary. The companion star, a more
or less normal star like our Sun, loses material onto the white dwarf via
accretion. Since the white dwarf is very dense, the gravitational
potential energy is enormous, and it is converted into X-rays during the
accretion process. There are probably over a million such cataclysmic
variables in the Galaxy, but only those close to our Sun (about 100 of
them) have been detected in X-rays so far. This is because CVs are fairly
faint in X-rays; they are just above the coronal X-ray sources and far
below the X-ray binaries in terms of how powerful their X-ray emissions
are.

Galaxies

Galaxies consist of 3 kinds of material: gas, stars,
and dark matter (material that we know must exist because its gravity is needed
to hold the galaxy together, but we can't observe it directly). When 2 galaxies
interact at a distance, they affect each other through their gravitational
forces. These are of 2 types: first is the ordinary gravitational attraction
which holds us onto the earth, and which holds solar system together; second is
the tidal force, which is due to the fact that gravity decreases with distance.
The tidal forces due to the moon and the sun are responsible for the earth's
tides. If they were very much stronger, they could actually rip the oceans off
of the earth, or rip the earth apart. There is no danger of this, but it can
happen in galaxy interactions, the tidal forces can disrupt one of the
galaxies, or remove the gas from one of them. This has been suggested as a way
of explaining why some galaxies (ellipticals) have little gas while others
(spirals) have a lot more. In this case some of the gas is probably transferred
to the bigger galaxy. Other possible interactions include total disruption of
one of the galaxies, or merging of the two galaxies. Which of these occurs
depends on how closely the galaxies approach each other and the masses of the
two galaxies.

A guide to some of the known galaxies:

Galaxy

Type

Distance (from Milky Way)

Size

Notes

Andromeda

Spiral

2,000,000 ly

200,000 ly

NGC 224

Centaurus A

Elliptical/ Oval

13,000,000 ly

180,000 ly

Black hole in centre.

Crab

Elliptical/ Circle

6500 ly

10 ly

Mostly destroyed by supernovas.
All stars are pulsars and neutron stars.